1. Field of the Invention
The present invention relates to a papermakers fabric, in general, and to a dryer felt constructed as a low permeability spiral fabric, in particular.
2. BACKGROUND OF THE PRIOR ART
A conventional dryer felt or fabric consists of an endless conveyor belt, typically made from a two, three or more plane fabric, wherein the various planes are defined by different groups of cross-machine direction yarns. The planes, plies, or layers, as they are variously called, are united by a plurality of machine direction yarns.
The yarns used to weave the most up-to-date dryer fabrics are generally made from synthetic monofilaments or synthetic multifilaments, from materials such as polyester, polyamide, acrylic and fiberglass. Dryer felts made predominantly from monofilament yarns have certain drawbacks. Because the monofilament yarns are relatively stiff, they are not easily bent around each other during the weaving process. Thus, the fabric that results has a relatively open structure. There are several positions on the papermaking machine that do not run or cannot run effectively when employing a very open fabric because of numerous problems with the paper sheet, such as thread-up, blowing, flutter which causes sheet breaks, and reeling problems.
A number of attempts to reduce the openness or permeability of dryer fabrics made predominantly of monofilaments have been tried. The major approach has been to use a bulky spun yarn as a stuffer pick in the middle of the weave pattern. These stuffer picks are, in effect, surrounded by the original monofilament cross-machine direction picks that are positioned in both the face and back surfaces of the fabric. This approach has been successful in reducing permeability, but has added little or nothing to the stability of the fabric. It has also created the disadvantage that the spun stuffer pick is prone to collect dirt. Also, the stuffer picks have a tendency to retain and carry moisture, a condition which is usually undesirable.
A second approach has been to modify the woven structure in such a way that the top, or face, cross-machine direction picks are offset in relation to the bottom, or back, cross-machine direction picks. Although this approach has produced relatively low permeability in an all monofilament fabric, there is no easy way to change permeability. The weave design does not permit the use of stuffer picks. Changes in yarn diameter are, of course, possible, but such changes can only be made within the limitations of the loom.
Yet another example of a way to control permeability in a dryer felt is the incorporation of warp yarns of rectangular cross section into a weave pattern that does not include provision for stuffer picks. In such a weave pattern, the warp or weft yarn typically floats on the paper-receiving surface of the fabric over a number of weft picks or warp ends. The longer the float, i.e., the more picks the warp yarn crosses, or the more ends the weft yarn crosses, before weaving back into the fabric, the less stable the fabric becomes. In this way, there is a tradeoff between permeability and fabric stability.
In addition to woven fabrics, certain types of non-woven fabrics have been employed as dryer felts or fabrics. Of particular interest to the present invention are those made from cross-machine direction spiral coils that are intermeshed and joined together by cross machine direction hinge yarns to create the machine direction of a dryer fabric of desired length.
As is presently known, the predominant approach to reducing permeability in spiral fabrics involves filling the gap within a given spiral coil created when that spiral coil is secured by hinge yarns to two adjacent spiral coils. Typically the gap is filled with a stuffer-type yarn. Another approach uses smaller spirals in an attempt to reduce the size of the space within a given coil.
In the first approach, the stuffer yarns are usually inserted as an extra production step after the basic fabric has been manufactured and finished. Although permeability is reduced, fabric processing time is increased and, therefore, this approach is less economical. At the same time, the use of stuffer yarns tends to reduce the clean running of the fabric and also reduces its ease of cleaning as dirt will rapidly adhere to the stuffer yarn.
The use of smaller spirals, on the other hand, necessitates increasing the number of filling yarns (which act as hinge yarns) per unit length. This again reduces productivity and increases costs. In addition, it has been observed that the reduction in permeability is relatively small, such fabrics, at best, having 800 cfm or more.
There is thus a need for a dryer felt of spiral coil construction that may be easily and economically produced to provide a wide permeability range that is stable and also dirt resistant, and that exhibits reduced moisture carrying properties. The present invention is directed toward filling that need.